The gas cylinders in question are made from metal, for example steel, starting from an unrefined piece cut from a sheet metal reel, from a tube or from a billet and subjected:                to a plurality of transformation steps that modify both the shape and the mechanical characteristics of the piece so as to obtain the gas cylinder with the desired shape and material characteristics and        to a plurality of checking and selection steps to identify and/or discard the pieces the checked characteristics of which do not correspond to predetermined target characteristics and        optionally, to assembly steps to complete the gas cylinder, for example the application of accessories or filament winding and curing in the case of a composite gas cylinder and        optionally, to finishing steps, for example the painting of the finished gas cylinder.        
In order to be able to find out the production batch of the unrefined material (reel of sheet metal, tubes or billets usually supplied by an external supplier to the manufacturing premises of the gas cylinders), it is known to equip all unrefined pieces of a production batch before the transformation steps with an initial marking comprising a stamped alphanumeric batch code that identifies such a production batch.
It is also known to equip each individual gas cylinder manufactured, after the transformation steps, with an individual final marking that comprises a stamped alphanumeric identification code that identifies the individual gas cylinder.
During a checking and selection step, for example during a hydraulic or volumetric expansion test of a gas cylinder, it is thus possible to store the identification code of the cylinder in a database and associate the result of the test with such an identification code.
The known method and system thus allows both the production batch of the unrefined material and the results of the hydraulic test to be traced and associated with each individual gas cylinder.
Indeed, in the technical field of gas cylinders it is believed that the technical characteristics of the individual gas cylinder depend mainly upon the characteristics of the unrefined material, whereas the influence of the individual metallurgic transformation steps (heat treatments and deformations) can be summarily considered through the outcome of the suitability tests, for example the volumetric expansion test, carried out on the finished gas cylinder.
Moreover, in the state of the art, it is not considered necessary to identify the individual unrefined piece or the individual semi-worked cylinder during the metallurgic transformation steps, since all of the pieces from the same production batch are considered to be the same and subjected to the same process.
Whilst we acknowledge this commonly held opinion, according to which the influence of the individual metallurgic transformation steps, i.e. the heat treatments and deformations, can be summarily considered through the positive or negative outcome of the suitability tests of the gas cylinders, the author of the present invention believes that this is a technical prejudice that, as a consequence, prevents it from being possible to identify or group gas cylinders already in use according to more specific and focused criteria of just the production batch of the unrefined material.
Indeed, in the hypothetical case of withdrawal from service of gas cylinders mounted for example in gas-powered vehicles or else in other uses, it would currently be necessary to extend the withdrawal to all the cylinders of an entire production batch of the unrefined material, whereas it would be useful to be able to limit the total number of cylinders involved based on a more detailed “manufacturing history” of each individual gas cylinder.